Facebook (Nasdaq: FB) shares are lower on the session following a volume move below the $19.50 level today. Overall, about 8.8 million shares have traded hands, from the three-month daily average of 45 million to 46 million shares.

A massive block of about 880,000 shares of Facebook were sold just after 10:00am EDT Tuesday, pushing shares below the support level. Since then, Facebook has gained some back and are currently just 0.5 percent lower.

Only about 78 million shares are being held short, a fraction of the 1.3 billion float and 2.14 billion outstanding shares.

Despite the drop, shares are still well above the $17.55 post-IPO low hit in September, when investors kept questioning how and when Facebook was going to convert mobile traffic into sales.